Quote Originally Posted by it-ogo View Post
Indeed 'да' instead of 'и' can often provide a speech with a slight archaic or folk color. But it is not so obsolete in modern language. Definitely not of 'thou art' level. Sayings and expressions are many rather than few. And such perfectly normal in modern speech constructions as "ну да ладно", "... да еще ..." etc. are more than just archaic set expressions IMO.

The truth is that it usually adds some colloquial and emotional color.
I doubt "да" has shade of "и" in "ну да ладно". For me it is "ну ладно" --> "ну да ладно". An emphatic particle and nothing more. "Ну и ладно" expresses a diffent emotion.